Transmission apparatuses such as a base station and the like, which transmit and receive signals, use amplifiers. The amplifiers use amplifying elements such as a field effect transistor (FET) and the like. The amplifying elements amplify signal in response to gate voltages applied to gate terminals. While currently, as such amplifiers, amplifiers such as an LDMOS-FET, a GaAs-FET, and the like have been used, recently there have been becoming more popular amplifiers utilizing highly-efficient gallium nitride (GaN)-FET devices capable of operating with lower power consumption than such amplifiers. Usually, an amplifier utilizing the GaN-FET device in this way amplifies a signal with the gate voltage of the GaN-FET device being fixed.